Fully evaluation nested binary operations

This PR request is an evolution of https://github.com/sass/libsass/pull/770 which cover many more cases where division operations aren't correct evaluated.

Address parts of https://github.com/sass/libsass/issues/629. Specs added https://github.com/sass/sass-spec/pull/201.

I'm going to leave this until 3.1.0 is stable because there's a chance this introduces subtle regressions not covered by our spec suite.